The present invention relates to a method and an arrangement for burning lime mud into lime in a lime kiln wherein the lime mud flows counter currently in relation to flue gases from a feed end into a firing end and wherein fuel gas is used as fuel, which gas is produced by means of gasification of a fuel in the presence of combustion air in a gasifier.
Typically, the lime mud formed in a recovery process of the chemical pulp industry is burned into lime (calcium oxide) and carbon dioxide in a rotary kiln. Other lime burning methods exist, but the quality of the lime produced and the combustion costs have not been proved as advantageous as those of lime produced in a rotary kiln.
The lime mud, i.e. calcium carbonate, fed into a rotary kiln is preheated with thermal energy of the flue gases in the kiln. Due to tilting and rotary motion of the kiln, the preheated lime mud flows into a combustion i.e. reaction zone located in the lower part of the kiln, where the calcination reaction required for the production of lime takes place. The last zone is a cooling zone where the lime is cooled prior to discharging it from the kiln.
The surface temperature of the lime in this so-called combustion zone is typically 1000-1200° C. The calcination reaction is strongly endothermal. For maintaining the temperature and for proceeding with the reaction, introduction of energy into the combustion zone is required. Combustion of lime mud into lime (calcium oxide) requires energy in the amount of usually 5500-500 MJ/t. For the production of thermal energy the kiln is provided with a burner, wherein fuel is combusted in the combustion zone. The temperature of the burner's flame is to clearly exceed the temperature of the lime in order to ensure that the energy required for the calcination reaction is transferred to the surface of the lime and therefrom into the core of the lime pellet, providing a satisfactory reaction result.
Sources of heat used in lime kilns comprise liquid, gaseous and solid fuels, such as oil, natural gas and carbon dust (coke). The burner is attached to a firing end in the hot end of the kiln.
The heat contained in the product produced in the lime kiln is recovered by means of transferring it into combustion air required in the burning of the fuel used in the process. Thereby this air (so-called secondary air) is usually led into the kiln bypassing the burner and only so-called primary air is led through the burner, which primary air is required for ignition, stabilization and formation of the flame. The portion of primary air varies depending on the burner and application in question, but most often it is 10-40% of the total amount of combustion air. The primary air is led into the burner via a dedicated fan.
The burnt lime coming from the rotary kiln is typically cooled in satellite or sector coolers rotating together with the kiln. More rarely the cooling takes place in a separate cooling drum or another separate cooling device. Lime mud combustion is characterized by high consumption of energy and thus high combustion air requirement. Thus, the cooling of the lime to an adequately low temperature can be effected only by means of the secondary air in the kiln.
U.S. Pat. No. 4,745,869 discloses a method, by means of which coal is used as fuel. A problem in the use of coal is that molten slag generated in the combustion adheres in the interior of the lime kiln and forms rings that may clog the flow in the kiln. According to the patent, this can be avoided so that a two-stage coal combustion chamber is connected to the kiln, from the first stage of which chamber the molten slag can be removed. The fuel gas generated in the first stage is combusted in the latter stage of the chamber. Combustion air required in the coal combustion chamber can be preheated with flue gases from the lime kiln in an indirect heat exchanger or with hot lime product of the kiln in a direct heat exchanger.
With high-energy fuels, such as oil, natural gas or high-quality coal, an adequately high temperature of the burner flame is easily reached. With wood gas or most of other gases produced by gasifying biomass in a gasifier this so-called adiabatic flame temperature instead remains below the desired value. This results e.g. in increase in the specific heat consumption in lime combustion, since it is not possible to utilize the energy content of the fuel to an adequately large extent for promoting the calcination reaction. The energy shortfall is corrected by burning more fuel, and as a result of that the feed end of the kiln is heated. Then it is not possible to utilize all the heat of the flue gas in lime mud preheating, but an increased portion of the heat escapes the process due to the increased temperature of the flue gas. When the adiabatic temperature of the flame remains low, the nominal production of the kiln can in an extreme case be unreached, since calcination does not proceed far enough with full-scale production. Further, increased flue gas amount can limit the production of the kiln when the auxiliary devices, such as a flue gas fan remain too small.
The product gas of gasification has been used as energy source for lime kilns since the end the 1980-es, but due to the low price of oil and mineral coal it has not gained high popularity. The increased prices of these fuels during the last few years have again made the product gas of gasification a fuel to be reckoned with. Wood bark and sawdust waste that are suitable fuels for a gasifier are readily available at chemical pulp mills.
Wood bark and corresponding biomass can be dried to typically 85% dry solids content, and the dried matter is gasified in e.g. circulating fluidized bed gasifiers at a temperature of typically 750-850 Celsius to product gas that contains as burning components carbon monoxide, hydrogen and hydrocarbons. Gas contains combustion products, such as carbon dioxide and water vapor, and also the moisture of the original dried biomass. Therefore the energy content of wood gas is not as high as that of typical main fuels: oil and natural gas.
In a gasifier, the air temperature required for burning is typically 20-400° C. Preferably it is preheated to approximately 300° C. or higher for minimizing the amount of gasification air. Thus, a higher heat value of the produced gas is achieved, as well as a higher combustion temperature in the lime kiln. By using preheated air, the carbon conversion of the gasifier, and thus the total efficiency is improved.
In the existing lime kilns using wood gas as fuel, cold air is used as combustion air in the gasifier, which air is preheated by means of the product gas from the gasifier, as presented e.g. by E. Kiiskilä (“Pyroflow gasifier replaces oil in lime kilns” in publication “Biomassan uusia jalostusmandollisuuksia 1990-luvulla” (New refining possibilities for biomass on 1990s); [VTT Symposium], 1987, Espoo, F I, Vol 75, pages: 76-89, and European patent application 2133402, whereby the temperature of the product gas is decreased. This temperature decrease directly reduces the temperature of the flame burning in the kiln, which is of importance. Alternatively the combustion air can be heated with steam exchangers, but also then valuable energy is to be consumed.
Compared to oil or natural gas lime kilns, the secondary air requirement of said lime kilns using gasification gas is smaller, since a portion of the fuel burning has already taken place in the gasifier. Thus, also the amount of air flowing through the lime cooler is smaller and the lime remains hotter, which means that the amount of thermal energy recovered therefrom back to the process therefrom is smaller than in oil or natural gas kilns. When using fuel gas of the gasifier, the fuel in the lime kiln already contains more oxygen, whereby its combustion air requirement is smaller anyway.